1. Field of the Invention
The present invention relates to improved methods of preventing water and/or gas flow through a subterranean zone, and particularly to such methods wherein the subterranean zone has a high temperature and is at a depth requiring a long pumping time.
2. Description of the Prior Art
The production of water with oil and/or gas from wells constitutes a major problem and expense in the production of oil and gas. While oil and gas wells are usually completed in hydrocarbon producing formations, when there is a water producing zone adjacent to the hydrocarbon producing formation, the higher mobility of the water often allows it to flow into the hydrocarbon producing formation by way of natural fractures and high permeability streaks. In the production of such wells, the ratio of water to hydrocarbons recovered often becomes so high that the cost of producing the water, separating it from the hydrocarbons and disposing of it represents a significant economic loss.
In order to reduce the production of undesired water and/or gas from hydrocarbon producing formations, aqueous polymer solutions containing cross-linking agents have been utilized heretofore. Such aqueous polymer solutions have been pumped into the hydrocarbon producing formations so that they enter water and/or gas zones within and adjacent to the formations and cross-link therein. The cross-linking of the polymer solutions causes them to form stiff gels which stop or reduce the flow of the undesired water and/or gas.
While the use of aqueous polymer solutions for reducing the production of undesired formation water and/or gas has achieved varying degrees of success, the cross-linking agents used to cross-link the polymer have often contained chromium or phenol/formaldehyde. These cross-linking agents are undesirable in that they either have high toxicities or produce cross-linked gels having low gel stabilities at high temperatures and/or at pH values greater than about 8.
In addition, the aqueous polymer compositions utilized heretofore for reducing the production of undesired water and/or gas from subterranean zones having high temperatures and requiring long pumping times have often been unsuccessful. That is, the compositions have not produced gels which remain stable at the high temperatures involved and/or they have not had the required pumping time at the high temperatures to reach and be placed in the subterranean zone.
Thus, there is a continuing need for improved methods of preventing water flow through subterranean zones.
The present invention provides improved methods of preventing undesired water and/or gas flow through a subterranean zone penetrated by a well bore. The subterranean zone can have a high temperature and be at a depth such that a long pumping time is required to place a sealing composition therein. The methods basically comprise preparing a polymeric sealing composition comprised of water, a cross-linking agent and a selected water soluble polymer or mixture of polymers which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the subterranean zone and has a sufficient pumping time before gelation in the presence of the cross-linking agent whereby the composition can be pumped to the depth of the zone and placed therein. Thereafter, the sealing composition is pumped into the zone by way of a well bore penetrating it and the sealing composition is allowed to form a cross-linked gel in the zone.
Depending upon the temperature of the subterranean zone to be sealed and the pumping time required, a selected type of water and a selected water soluble polymer are used. The water is selected from natural seawater, synthetic seawater, brine, unsaturated salt water and fresh water. While various water soluble polymers can be utilized, suitable such polymers are polyacrylamide, AMPS(copyright)/acrylamide copolymers, sulfonated styrene/maleic anhydride copolymers, vinylpyrrolidone/AMPS(copyright)/acrylamide terpolymers and AMPS(copyright)/N-N-dimethylacrylamide/acrylamide terpolymers.
The cross-linking agent utilized in accordance with this invention is preferably chitosan which is a biodegradable and non-toxic glucosamine polymer derived from chitin.
In order to reduce the pumping time of a sealing composition of this invention while maintaining the temperature at which a stable gel is formed, a gelation accelerating agent can be included in the sealing composition. The weight ratio of polymer to cross-linker can also be varied to lengthen or shorten the pumping time.
Thus, it is a general object of the present invention to provide improved methods of preventing undesired water and/or gas flow through a subterranean zone.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
The present invention provides methods of preventing undesired water and/or gas flow through a subterranean zone penetrated by a well bore. As mentioned, the subterranean zone can have a high temperature and be at a depth such that a long pumping time is required to place a sealing composition therein. The term xe2x80x9chigh temperature(s)xe2x80x9d is used herein to mean a temperature in the range of from about 150xc2x0 F. to about 500xc2x0 F. The term xe2x80x9clong pumping time(s)xe2x80x9d is used herein to mean a pumping time in the range of from about 2 hours to about 14 days. The xe2x80x9cpumping timexe2x80x9d is the length of time between when a polymeric sealing composition of this invention is prepared and when the composition forms a gel and can no longer be pumped into or through a porous rock formation without fracturing the formation.
The methods of this invention basically comprise the steps of preparing a polymeric sealing composition comprised of water, a cross-linking agent and a selected water soluble polymer or mixture of polymers which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the zone. The polymeric sealing composition also has a pumping time before gelation in the presence of the cross-linking agent whereby the composition can be pumped to the depth of the zone and placed therein. After preparation, the sealing composition is pumped into the zone by way of the well bore penetrating it and is allowed to form a cross-linked sealing gel in the zone.
It has been discovered that the temperature at which gelation occurs, the length of time the resulting gel is stable and the pumping time of an aqueous polymer composition containing a cross-linking agent can all be controlled by the proper selection of the water and the polymer or polymers utilized in the composition. The water can be selected from natural seawater, synthetic seawater, brine, unsaturated salt water and fresh water, and the polymer or polymers can be selected from homopolymers, copolymers, terpolymers and mixtures thereof. By increasing the level of bulky monomers in the selected polymer or polymers utilized, the temperature at which the polymer compositions form a gel can be raised, the pump times can be increased at a particular temperature and/or the stability of the gel at a particular temperature can be reduced or increased. A relatively short gel stability can be utilized for temporarily sealing a subterranean zone while a gel having long term stability can be used for sealing a subterranean zone for an extended period of time.
In addition to the particular types of water and polymer utilized in the sealing composition, the gel time and gel stability can be varied by varying the polymer to cross-linking agent weight ratio in the sealing composition and/or including a gelation accelerating agent in the composition.
The sealing compositions useful in accordance with the present invention are comprised of water, a cross-linking agent and a selected water soluble polymer or mixture of polymers which reacts with the cross-linking agent and forms a sealing gel that has a desired pumping time before gelation and is stable for a desired period of time at the temperature of the subterranean zone in which it is placed.
The water in the sealing compositions can be natural seawater, synthetic seawater, brine, unsaturated salt water or fresh water. The particular type of water utilized changes the pumping time of the sealing compositions before gelation occurs. Generally, the pumping time increases with increasing salt concentration in the water. When chitosan is utilized as the cross-linking agent, the water can include an organic or inorganic acid to facilitate the dissolution of the chitosan.
The preferred cross-linking agent for use in accordance with this invention is chitosan which is biodegradable and non-toxic. Chitosan is a polymeric glucosamine derivative of chitin which is a naturally occurring glucosamine polysaccharide structurally similar to cellulose. Chitin is the principal constituent of the shells of crabs, lobsters and beetles.
Chitosan polymers have heretofore been cross-linked with cross-linking agents such as glyoxyl, glutaraldehyde, oxo-acids and acid anhydrides and utilized as highly absorbent materials (see U.S. Pat. No. 5,599,916 issued to Dutkiewiez et al. on Feb. 4, 1997), gels having high adsorbing power (see U.S. Pat. No. 4,835,265 issued to Muzzarelli on May 30, 1989), highly enriched magnetized particles (see U.S. Pat. No. 5,864,025 issued to Glasser et al. on Jun. 26, 1999) and other similar materials (as taught by Roberts and Taylor in the Journal of Applied Polymer Science, Vol.31, pp. 1169-1176 [1986]).
A variety of water soluble polymers can be utilized in accordance with the present invention. Examples of suitable such polymers include, but are not limited to, polyacrylamide, polyvinyl pyrrolidone AMPS(copyright)(2-acrylamido-2-methylpropane sulfonic acid)/acrylamide copolymers, sulfonated styrene/maleic anhydride copolymers, vinylpyrrolidone/AMPS(copyright)/acrylamide terpolymers, AMPS(copyright)/N-N-dimethylacrylamide/acrylamide terpolymers, polyketones and oxidized polyvinyl alcohol. The water soluble polymers which are used in the present invention can themselves be biodegradable and/or non-toxic. Examples of such polymers include alginic acid derivatives, oxidized polyvinyl alcohol and low molecular weight acrylic polymers.
Generally, by increasing the fraction of bulky or less reactive monomers in the polymer, the temperature at which gelation occurs can be increased, the pumping time at a given temperature can be increased and/or the stability of the gel formed can be increased or decreased. For example, when a subterranean zone to be sealed has a temperature in the range of from about 70xc2x0 F. to about 230xc2x0 F. and requires a pumping time in the range of from about 2 hours to about 96 hours, the water soluble polymer utilized in the sealing composition can be polyacrylamide or AMPS(copyright)/acrylamide copolymer. When the subterranean zone has a temperature in the range of from about 200xc2x0 F. to about 350xc2x0 F. and requires a pumping time in the range of from about 2 hours to about 14 days, the water soluble polymer can be a sulfonated styrene/maleic anhydride copolymer, a vinyl pyrrolidone/AMPS(copyright)/acrylamide terpolymer, an AMPS(copyright)/N-N-dimethylacrylamide/acrylamide terpolymer or a mixture of such polymers.
While biodegradable and non-toxic chitosan is preferred for use as the organic cross-linking agent in accordance with this invention various other organic cross-linking agents can be utilized in accordance with the present invention. Examples of such organic cross-linking agents are polyalkyleneimines, polyalkylenepolyamines and mixtures thereof. In addition, water-soluble polyfunctional aliphatic amines, arylalkylamines and heteroarylalkylamines can be utilized.
Generally, the water soluble polymer utilized is included in a sealing composition of this invention in an amount in the range of from about 0.5% to about 20% by weight of the composition. The cross-linking agent is included in the sealing composition in an amount in the range of from about 0.05% to about 15% by weight of the composition. The weight ratio of the water soluble polymer to a chitosan cross-linking agent is in the range of from about 50:1 to about 1.1:1.
As mentioned above, the weight ratio of water soluble polymer to cross-linking agent can be varied to change the pumping time before gelation and other properties of the sealing composition. That is, the weight ratio of the water soluble polymer used to the cross-linking agent can be varied in the range of from about 10:1 to about 400:1 to achieve desired results.
The gelation accelerating agent mentioned above which can be utilized to reduce pumping time before gelation at a given temperature can be a pH control compound such as an alkali metal carbonate, bicarbonate or hydroxide, a mineral acid such as hydrochloric acid, an organic acid such as acetic acid, a Lewis acid such as boric acid or other compounds such as ammonium chloride, urea and lactose. Of these, boric acid is preferred. When utilized, boric acid is added to the sealing compositions of this invention in a general amount in the range of from about 0.005% to about 0.1% by weight of the composition.